Marine Invertebrates of the Andaman and Nicobar Islands

About 50 species of marine sponges occur in these Islands and only a few of them have been investigated. Even their classification is in a confused state. Several bioactive metabolites of considerable interest have been isolated from Agelas species182 183 found in the Andaman and Nicobar Islands. A. nata the only species reported to occur remains to be investigated. Four species of Cliona, C. ensifera, C. lobata, C. mucronata and C. quadrata occur in the Andaman Islands. However, none of these appear to have been investigated. Two species of genus Craniella, C. cranium and C. dactyloides that are found in the Islands have remained uninvestigated. Phyllospongia foliascens Pallas which occurs in the Islands has furnished foliaspongin,184 an antiinflammatory bishomosesterpene. An antiallergic and antiinflammatory glycolipid has been isolated from P. foliascens.185 Chemical investigation of Petrosia testudiana is expected to furnish compounds of biological interest. Two species of genus Tedania, T. nigrescens and T. brondifera are found in the Islands and so far these are not investigated. However, these species may yield metabolites of biomedical interest.

6.1 Coelenterates

There has been much interest in the metabolites of jelly fish, sea nettle, the Portuguese man-of-war and the sea wasp which are widely distributed in warm tropical seas. The organisms release a nematocyst venom from the tentacles which causes painful injuries. The venoms are generally a complex mixture of enzymes and pain-producing factors. The nematocyst venom of P. physalis is a mixture of toxic proteins and enzymes which show multiple action including dermonecrosis, neurotoxicity, hemolysis and cardiotoxicity. The cardiotoxin from the wasp has been purified by immunochromatography.

6.2 Sea Anemones

Several species of sea anemones occur in the sea coasts of the Andaman and Nicobar islands. The toxins produced by these organisms are polypeptides or proteins. The toxins are found very useful tools for studying the voltage dependent Na+ channels in nerve and cardiac muscle cells. It has been suggested that coelenterate toxins would be suitable for studies of tumor cell cytolysis in vitro and in vivo.

6.3 Corals

Some of the soft corals that occur in the Andaman and Nicobar Islands are horny gorgonians, sea fans, the red organ-pipe coral and the blue coral, Helipora. The mushroom coral Fungia is the most common solitary coral that occur in the Andaman and Nicobar islands. The coral Clavularia viridis has yielded cytotoxic steroids, stoloniferones A-D.186 Muricea species such as M. californica and M. fruticosa have provided four new esterified aminogalactose saponins called muricins187 which inhibit the growth of the diatom Phaeodactylum tricornum at 100 ppm concentrations. It is suggested that this activity may play a significant role in reducing fouling due to the diatoms. Soft corals elaborate a large variety of sesquiterpenoids and diterpenoids. Several of these are found to be toxic. Guaiazulene from the gorgonian Euplexaura erecta1&& exhibits mild activity against Pseudomonas aeruginosa. Subergorgic acid, a cardiotoxin is obtained from the pacific gorgonian coral Subergorgia suberosa.m The toxin inhibits neuromuscular transmission at 0.16 ^g/mL in isolated guinea-pig heart assay. Pseudopterolide, an unusual diterpene with a 12 member ring from the gorgonian Pseudopterogorgia acerosa190 shows unusual cytotoxic properties.

6.4 Bryozoans

Marine bryozoans constitute one of the important groups of fouling communities. Bugula spp occur abundantly on the ship bottoms. B. neritina has furnished several new macrolides called bryostatins.191-197 Bryostatin-4 was isolated from B. neritina, bryostatin-8 from Amanthia convoluta.192 The origin of bryostatin-5 and bryostatin-6 is uncertain. The marine bryozoan, Flustra foliacea191 has yielded several brominated alkaloids called flustramines

A, B, C, flustraminol A, B and dihydroflustramine C, which inhibits cell division of the fertilized sea urchin eggs. The bryozoan Phidolopora pacifica has yielded phidolopin, a purine derivative largely responsible for high order of antifungal and antialgal activities.198,199 Several macrolides like bryostatin-1 and bryostatin-2 were isolated from Bugula neritina.200'202 Some of these metabolites show high order of antineoplastic activity. [2-Hydroxyethyl] dimethyl sulfoxonium ion acts as an allergen. These compounds are isolated from marine bryozoan Alcyonidium gelatinosum.203 The bryozoan causes an eczematous allergic contact dermatitis called 'Dogger bank itch'.

6.5 Molluscs

Several steroidal, terpenoids, and acetylenic compounds isolated from nudibranchs were also found in sponges which they feed upon.204 The bioactive nucleoside characterised as 1-methyl-isoguanosine has been found in the sponge Tedania digitata205,206 as well as in the nudibranch Anisodoris nobilis.207-209 Isoguanosine isolated from marine nudibranch Diaulula sandiegensis210 produced hypotension and relaxation of smooth muscles in mammals. Hexadecylglycerol isolated from Archidoris montereyensis 211 showed high order of activity in vitro against Staphylococcus aureus and Bacillus subtilis. Sea-hares accumulate large quantities of metabolites in their digestive gland and skin. These compounds are believed to originate from the algae which they take as food. Aplysiatoxin, a toxic metabolite has been isolated both from the Hawaiian sea-hare Stylocheilus longicauda and also from blue-green alga Lynbrya majusticula212 on which it feeds. Aplysistatin is a well known antileukemic metabolite from the sea-hare Aplysia angasi213 The metabolites of A. dactylomela are reported to have cytotoxic and antitumor activity in vivo.214 Dolabella auricularia has yielded several antineoplastic and cytotoxic compounds named dolastatins.215-219

Marine snails of the family conidae synthesize potent toxins which they inject into their prey by means of a hollow tooth in order to immobilise their prey. Some species are known to cause injuries to humans and have proved to be fatal. The venom of Conus geographus is most dangerous to man. Striatoxin, a cardiotonic glycoprotein obtained from C. striatus220 was found to have long lasting inotropic action on guinea-pig left atria. Its minimum lethal dose in the fish Rhodeus ocellatus was 1 ^g/g body weight. KelletininI and II isolated from marine mollusc, Kelletia kelletii221 inhibit the growth of Bacillus subtilis and L1210 leukemia cells. Surugatoxin and neosurugatoxin are isolated from Babylonia japonica (Japanese ivory shell).222'223 The antinicotinic activity of the latter is found to be 100 times that of the former. Siphonaria diemensis has furnished the antibiotic diemensin A and diemensin

B.223 The former inhibits the growth of Staphylococcus aureus and Bacillus subtilis at 1 ^g/disc and 5 ^g/disc, respectively. It also inhibits cell divisions in the fertilized sea-urchin egg assay at 1 ^g/mL.

6.6 Echinoderms

The metabolites of Echinoderms mainly responsible for the biological activity are saponins. The chemistry and biological activities of bioactive saponins have been reviewed.224 Asterosaponins are reported to have hemolytic, antineoplastic, cytotoxic, antitumor, antibacterial, antiviral antifungal and antiinflammatory activities.225,226 Saponins of sea-urchins and starfishes differ considerably. Asterosaponins are sterol derivatives, whereas sea cucumber saponins are terpenoid in nature. Both groups have sulphate esters and quinovose sugar moieties. However, in asterosaponins the sulphate function is attached to an aglycone, whereas it is attached to the carbohydrate moiety in some sea cucumber saponins. The saponins from other sources lack sulphate functionality. To date, the saponins of over 50 sea cucumbers have been studied.227 Many cerebrosides, pyrimidine nucleosides, thymine deoxyriboside and uracil deoxyribose have been isolated from the starfish Acanthester planei.228

6.7 Sea-urchins

Pedicellaria of some species of sea-urchins contained toxic substances. Extract of Toxopneustes pileolus causes histamine release from isolated smooth muscles.229 The extract of the organism produced contraction of the longitudinal muscles of isolated guinea-pig ileum at a concentration of 3 x 10-8 g/mL. Lytechinus variegatus and Strongylocentrotus droebachiensis have yielded antineoplastic glycoproteins.

6.8 Tunicates

The most interesting compounds of Tunicates are the cyclic oligopeptides. Lissoclinum patella has furnished several cyclic peptides.232 233 Of these ulicyclamide, ulithiacyclamide, and patellamide A, B, and C exhibit antitumor activity against L1210 murine leukemia culture in vitro. A new class of depsipeptides, some of them exhibited high order of antiviral (against RNA and DNA viruses) and antitumor (against L1210; P388 leukemia and B16 melanoma) activities, are obtained from Trididemnum species of the family Didemnidae.234,235 The compounds designated as eudistomins contain substituted condensed oxathiazepine ring system. These compounds show high order of antiviral activity against Herpes simplex virus type (HSV-1) and have been isolated from the colonial caribbean tunicate Eudistoma olivaceum.236 Other eudistomins having substituted P-carboline system and displaying modest activity against HSV-1 and Bacillus subtilis have been isolated from the same species.237

Dolastatin 10, a novel pentapeptide isolated from the marine mollusk Dolabella auriculata is in phase II clinical trials. Further studies revealed that it is unlikely to have substantial activity in the treatment of melanoma.238 Aurantosides D, E and F exhibiting high order of antifungal activity against Aspergillus fumigatus and Candida albicans were isolated from marine sponge

Siliquariaspongia japonica.239 Two chemotypes of marine bryozoan Bugula neritina were identified.240 A new polysaccharide exhibiting anti-HIV activity and made up of galactan sulfate was isolated from the marine clam Meretrix petechialis241 Dolastatin 15, a potent antineoplastic peptide from the mollusk Dolabella auriculata was undergoing clinical trials in Europe and North America.242 Two sesquiterpenes active against phytopathogenic fungus Cladosporium cucumerinum were isolated from a marine soft coral of the genus Heteroxenia243 A stereocontrolled synthesis of the antitumor agent (-)-laulimalide was achieved.244 Linckosides A and B, the new neuritogenic steroid glycosides were isolated from the Okinawan starfish Linckia laevigata.245 Three alkaloids having an unusual decahydroquinoline skeleton and showing significant and selective antiplasmodial and antitrypanosomal activities, were obtained from a new tunicate species of the genus Didemnum.246 These bioactive alkaloids may serve as lead structure for the development of new antimalarial drugs. Vitilevuamide, a bicyclic peptide was isolated from marine ascidians Didemnum cuculiferum and Polysyncranton lithostrotum. The peptide was cytotoxic in several human tumor cell lines with IC50 values ranging from 6 to 311 nM.247 Several polyacetylenes exhibiting significant selective cytotoxicity against human tumor cell lines were isolated from the marine sponge Petrosia sp.248 Plakevulin A, a new oxylipin inhibiting DNA polymerases X and Y was isolated from the sponge Plakortis sp.249 A novel prodigiosin-like immunosuppresant was isolated from Micrococcus sp. It had considerable selective T cell immunosuppressive potential.250 Dolabellanin B2, a novel peptide consisting of 33 amino acid residues was isolated from the body wall of the sea hare Dolabella auricularia.251 It was effective against some pathogenic microorganisms at a 2.5-100 ^g/ml.251 A phase-II clinical study of bryostatin 1, in patients with advance lung cancer was carried out.252 Two new tryptase inhibitors cyclotheonamide E4 and E5 were isolated from the marine sponge of the genus Ircinia. Cyclotheonamide E4 showed potent inhibitory activity against human tryptase (IC50 5.1 nM) and may be useful as a therapeutic agent in the treatment of allergic diseases including asthma.253 Aplidin (R) (APL) a novel depsipeptide from the tunicate Aplidium albicans exhibiting high order of cytotoxic activity was under clinical studies. Sulfated polymannuroguluronate (SPMG), a marine sulfated polysaccharide has entered in phase II clinical trial in China as the first AIDS drug.254 Two new cytotoxic compounds, viz. nakiterpiosin and nakiterpiosinone were isolated from the Okinawan sponge Terpios hoshinota.255 Aplidine, a marine depsipeptide from Mediterranean tunicate Aplidium albicans was in phase-II clinical trial.256 Aplidine in human Molt-4 leukaemia cells was found to be cytotoxic at nanomolar concentrations and induced both a G(1) arrest and a G(2) blockade. The drug induced cell cycle perturbations and subsequent cell death do not appear to be related to macromolecular synthesis (protein, RNA, DNA) since the effects occur at concentrations (e.g. 10 nM) in which macromolecule synthesis was not markedly affected. Although the mechanism of action of aplidine is still not clear, the cell cycle phase perturbations and the rapid induction of apoptosis in Molt-4 cells appear to be a mechanism different from that of known anticancer drugs.256 A profile of the in vitro antitumor activity of lissoclinolide, a small non-nitrogenous lactone isolated from the marine ascidian Lissoclinum patella is available.257 Lissoclinolide had a moderate selective activity towards colon tumor cell lines. The sponges of Acanthella sp. are prolific source of highly functionalized diterpene antibiotics. Eight kalihinol type diterpenes were isolated from two A. cavernosa.258 Haliclona sp. a marine sponge yielded kendarimide A, a novel peptide which reversed glycoprotein mediated multidrug resistance in tumor cells.259

Xestospongin C and D isolated from marine sponge Haliclona exigua inhibited rat brain nitric oxide synthase activity.260 Lamellarin a-20 sulfate, an ascidian alkaloid inhibited HIV-1 integrase of virus in cell culture.261 Phase-I clinical trials were carried out on dolastatin-10, a peptide isolated from the marine mollusk Dolabella auricularia in patients with advanced solid tumor.262 Spongistatin 1, a macrocyclic lactone polyether from marine sponge Hyrtios erecta exhibited a broad range of antifungal activity against a variety of opportunistic yeasts and filamentous fungi.263

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